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«Abstract Tanks were an integral part of rural life in India traditionally. However, with decreasing collective action by the community inappropriate ...»

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Citation: Osman Mohammed, Wani, S.P., Vineela, C and Murali, R. 2009. Quantification of Nutrients

Recycled by Tank Silt and its Impact on Soil and Crop - A Pilot Study in Warangal District of Andhra Pradesh.

Global Theme on Agroecosystems Report no. 52. Patancheru 502 324, Andhra Pradesh, India; International

Crops Research Institute for Semi-Arid Tropics. 20 pages.


Tanks were an integral part of rural life in India traditionally. However, with decreasing collective action by the community inappropriate soil and water management practices adopted by the farmers, encroachments of tanks and waterway by the individuals resulted in neglect of the tanks in villages. Good practices such as desilting and application of silt to agricultural fields were abandoned. Continued mining by crops and reduced application of organic manures have resulted in deficiency of several nutrients particularly that of micronutrients. ICRISAT in association with Modern Architects of Rural India (MARI), an NGO conducted a pilot project and quantified major and micro-nutrients present in the tank silt and also its impact on soil health and crop yields.

The depth of silt in 12 tanks de-silted ranged from 1.2 m to 3.0 m. The pH of the tank silt ranged from 6.5 to 8.5, while the organic carbon content was found to be low (0.5% to 0.8%). The available N content of tank silt ranged from 328 mg kg-1 to 748 mg kg-1, available P 5 to 35 mg kg-1 and K 271 to 522 mg kg-1 silt.

Similarly, available S ranged from 12 mg kg-1 to 30 mg kg-1 zinc from1.2 mg kg-1 to 5.6 mg kg-1 and boron

0.4 to 0.8 mg kg-1 silt. Microbial population was found to be low and it could be due to excessive use of pesticides for cash crops like cotton and chilli grown in the catchment area. Textural analysis indicated 70 to 80% clay, while the silt ranged from 15 to 25%. Addition of tank silt at 50, 100, 150 and 375 tractor loads per hectare improved the available water content by 0.002, 0.007, 0.012 and 0.032 g g-1 of soil, respectively in the plough layer and enhanced the tolerance of rain-fed crops to moisture stress by three to five days.

The farmers could recover the investment made on transport of the silt through increased net profit in cot- ton and chilli compared to turmeric and maize. Further, the saving on pesticides alone was to the tune of Rs. 2500 ha-1 in cotton and chilli crops, which has indirect beneficial impact on the ecosystem. De-silting was found to be an economically viable activity both in terms of farmers’ and project’s perspective to create more storage capacity as well as to return the silt back to the fields. De-silting activity needs greater sup- port from the government and non-governmental agencies for achieving multiple outputs like employment generation for landless, rejuvenating of the tanks and for enhanced productivity of dryland crops.

Key words: tank silt, nutrient recycling, economic evaluation and impact assessment

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Quantification of Nutrients Recycled by Tank Silt and its Impact on Soil and Crop - A Pilot Study in Warangal District of Andhra Pradesh Mohammed Osman, Suhas P Wani, C Vineela and R Murali

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Acknowledgements We sincerely thank help of the farmers who conducted Participatory Research and Development (PR&D) trials. We also thank Drs KL Sahrawat and KV Padmaja for reviewing the manuscript; Ms N Shalini for editing the manuscript; Mr KNV Satyanarayana and Ms N Srilakshmi for page-setting and word processing; and Communication Office, ICRISAT for production of this report. We gratefully acknowledge the financial support provided by WWF International.

The opinions expressed in this publication are those of the authors and do not necessarily reflect those of ICRISAT or WWF International. The designations employed and the presentation of material in this publication do not imply the expression of any opinion whatsoever on the part of ICRISAT or WWF International concerning the legal status of any country, territory, city or area, or concerning the delimitation of its frontiers or boundaries. Where trade names are used, this does not constitute endorsement of or discrimination against any product by ICRISAT or WWF International.

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1. Introduction

2. Materials and Methods

3. Results

3.1. Chemical Properties

3.2. Physical Properties

3.3. Biological Properties

4. Impact on Soil

5. Impact on Crop Growth and Yield

6. Economic Evaluation

7. Policy Implications


Annexure - I


1.0 Introduction In southern India, community tank systems are integral part of rural livelihoods for centuries.

True to wetland ecosystem, the interactions between human, land and water are the highest in tanks and provides the highest productivity both in agriculture and ecosystem uses (DHAN, 2004). Tanks have multiple functions and several outputs like food (fish), fodder (tank bed) and fuel (bushes), ecosystem services like biodiversity (flora, fauna, avian), groundwater recharge and supporting services like washing, bathing, retting, etc., in addition to the main use as source of irrigation. Tanks serve as a common pool resource and have various stakeholders ranging from governmental agencies, local panchayats, farmers, rural rich and poor. The breakdown of traditional system has resulted in the encroachment, siltation, weed growth and poor inflows. Over exploitation of groundwater through bore wells have made these water bodies a neglected entity, truly as “tragedy of commons”. Poor management practices of catchment have resulted in silting of most of these water bodies and significant reduction of storage capacity. Silt deposit has not only reduced the storage capacity but also groundwater recharge, eutrophication of tanks and most importantly, higher release of carbon into atmosphere through silt mediated anaerobic decomposition of organic carbon.

Though tanks are in existence across the country, they have not figured in any national programs. It is conspicuous that there are no countrywide programs as that of the Command Area Development Program (CADP) and Integrated Watershed Development Programs (DHAN, 2004). Tanks having more than 40 ha of command area are entrusted to panchayats, which are struggling for mobilization of funds and are loaded with too many activities. Most of the budget outlay goes to major and medium irrigation projects at national and state level, while the minor irrigation projects receive step-motherly treatment, which involves less investment and yields higher returns. Tanks and ponds provide water where people need it and support biodiversity. One of the advantages of tank restoration is the equity as they are evenly distributed over the landscape unlike canals, which follow the gradient and irrigate mostly the richly-endowed areas.

Green revolution has virtually transformed ‘low external input’ into ‘high external input’ agriculture. Soil is considered as pool of nutrients present in both available and reserve forms. Depletion occurs when nutrients don’t get replenished from the reserve pool. Soil is not an eternal supplier of all the nutrients when exploited indiscriminately through excessive mining by crops or land degradation. Out of total 16 elements essential for plant growth, seven are required in much smaller quantities and are called micro-nutrients.

They are namely, iron, manganese, boron, zinc, copper, molybdenum and chlorine. In most soils, the deficiency of boron and zinc is widely noticed (Rego et al. 2005, Sahrawat et al. 2008).

Nitrogen (N), phosphorous (P), potassium (K), calcium (Ca), magnesium (Mg) and sulphur (S) are considered as macro-nutrients as their requirement by plants is high, particularly the first three (N, P and K). It is estimated that there is an annual depletion of about 5.8 million tonnes of major nutrients due to agricultural production system, mostly P and K since most farmers apply these nutrients in much lesser amounts than needed (Rajendra Prasad, 2002). The normal application ratio of NPK is 4:2:1, but is now heavily biased towards N, resulting in nutrient imbalances.


2.0 Materials and Methods The pilot study has been carried out in collaboration with Modern Architects of Rural India (MARI), an NGO active in Warangal district of Andhra Pradesh and was funded by World-Wide Fund for Nature (WWF). Warangal district lies between 170–19’ and 180–36’ North latitude and 780–49’ and 800–43’ East longitude. The elevation ranges from 266 m to 518 m MSL. On the north part of the district lies Karimnagar, West Medak and to South Nalgonda district and to East Khammam. The district falls in the catchment of both Krishna and Godavari rivers, two important rivers of Andhra Pradesh. The geographical area of the district is 12846 sq. km. About 41% of the total area is under cultivation, while 29% is under forest. Current and other fallow account for about 15% and the rest 15% is under miscellaneous category (non-agricultural, barren, grazing land, cultivable waste). All the mandals receive about 1000 mm rainfall, mainly through S-W monsoon.

The study was carried in four mandals of the district, which have high percent of cropped area under irrigation, namely Nalabelli, Parkal, Shayampet and Regonda through tanks and open dug/bore wells.

Salivagu micro basin of Godavari river having 447 tanks spread over 878.35 sq km of catchment was selected for the study. Twelve tanks were identified in the Salivagu micro basin for de-silting on pilot basis during 2005-06. Name of the village, tank and the number assigned to the tank is set out in (Table 1).

Samples of tank silt were drawn using 5 cm core from four layers (0 to 30 cm, 30 to 60 cm, 60 to 90 cm and 90 to 120 cm) at various locations in the tank bed area proposed for de-silting. A composite sample for each depth was drawn using normal sampling procedure. Various chemical, physical and biological parameters of silt were assessed using standard methods (Table 2).

The valuation of nitrogen in the silt was based on the cost of urea while phosphorous on the basis of single super phosphate (SSP). Potassium was based on muriate of potash (MOP), zinc (zinc sulphate) and boron (Borax) at the existing rates. Value of tank silt was based on the content of N,P,K, zinc and boron and equated with cost of fertilizers. The benefit was calculated by summing the value of silt for different nutrients. Value of other nutrients was not estimated. The benefit-cost ratio calculated was the apparent value and indicated only the cost of de-silting operation borne by the project and the total value of the nutrients.

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3.0 Results The depth of silt deposit in 12 tanks ranged from 1.2 m to 3.0 m. The depth was found to be the highest in Rayaparthi tank (T10) and the least in Repaka (T11).

3.1 Chemical Properties pH and electrical conductivity (EC): The pH of the tank silt ranged from 6.5 to 8.5. pH of the tank silt varied with depth (Table 3). Except Relakunta (T-2) all the tanks recorded pH 7.0 and above, while Rudragudum (T-3) recorded the highest (8.5). pH has high relevance and some crops are very sensitive. If soils with high pH receive more tank silt having high pH, it might affect the crop productivity adversely. EC was found to be normal (0.4 dS m-1) and within safe limits for all the tanks.

3 Nitrogen and organic carbon content: Available N-content of tank silt ranged from 328 mg kg-1 to 748 mg kg-1 silt. The organic carbon content was found to be low and ranged from 0.5% to 0.8%. The highest value of organic carbon content of 1.5% was recorded in Munchupla at surface while it declined with depth (Table 3). The quality of Munchupla (T-12) was found to be superior compared to other tanks in terms of normal pH, high organic carbon and nitrogen contents.

Phosphorous and potassium content: The available phosphorous content ranged from 5 mg kg-1 to 35 mg kg-1 while exchangeable K from 271 mg kg-1 to 522 mg kg-1 silt.

Sulphur, zinc and boron: Available S (12 mg kg-1 to 30 mg kg-1) zinc (1.2 mg kg-1 to 5.6 mg kg-1) and boron (0.4 mg kg-1 to 0.8 mg kg-1) were found to be highly variable.

3.2 Physical Properties All the tanks had high clay content, followed by silt, fine sand and coarse sand irrespective of the depth, indicating tank silt richness in clay than silt. All the tanks except Rudragudum (T-3) had 70% to 80% clay while silt ranged from 15% to 25% (Table 4). Fine sand and coarse together amounted to less than 10% of the total in all the tanks except T-3. High clay content at all the depths was noticed in case of Repaka (T-11), while high silt content was found in Gorikothapalli (T-5) at all the depths, except surface level (0 to 30 cm).

Depth-wise values of coarse sand, fine sand, silt and clay are indicated in Table 4.

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3.3 Biological Properties Microbial population (bacteria, fungi and actinomycetes) was found to be low due to crops like cotton and chilli in the catchment consuming large amount of pesticides. The bacterial population of tanks in Medak varied from 200 x 103 CFU g-1 to 300 x 103 CFU g-1 (Padmaja et al. 2003) when compared to low counts of 0.2 x 103 CFU g-1 and high counts of 92 x 103 CFU g-1 for tanks in Warangal district (Fig. 1). Microbial biomass C ranged from 204 to 383 µg C g-1 soil, while the microbial biomass N ranged from 19 to 31 µg M g-1 soil.

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60 40 20

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Figure. 1 Microbial population of different tanks 8

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